1. Field of the Invention
This invention relates to an outboard engine mounted to a boat""s stern with a mounting device having a tilt shaft, and more particularly, to a structure related to a return oil path for returning lubricant oil to an oil pan after lubricating portions of an engine to be lubricated.
2. Description of the Related Art
Heretofore, lubricant oil discharged from an oil pan in an outboard engine has been returned to the oil pan located at a lower portion of an engine body through a return oil path after lubricating some portions of the engine to be lubricated. Regarding such a return oil path, in an outboard engine disclosed in Japanese Patent Laid-Open Publication No. hei 7-149290, for example, an opening is provided in an occlusive plate forming the bottom wall of the engine block of the engine having a vertically extending crankshaft, such that return oil flowing from the crank chamber onto the occlusive plate can drop into the oil pan through the opening through an oil communication path formed in an engine mount case. Below the occlusive plate, a flywheel is disposed, which is fixed to a lower end portion of the crankshaft extending through the occlusive plate, covered by the occlusive plate thereabove, and surrounded by the circumferential wall of an engine mount case and an encircling wall. The oil communication path is formed between the encircling wall that is one of the circumferential wall and the encircling wall located behind and another circumferential wall located behind the encircling wall with a distance, and the opening is formed at a rear portion of the occlusive plate opposite from the flywheel located forward with respect to the encircling wall.
In the conventional outboard engine, the opening defining the return oil path for returning the lubricant oil accumulating in the crank chamber to the oil pan is located at a rear portion of the crank chamber located above the flywheel. Therefore, if the outboard engine is driven in a tilt-up condition during cruising in shallow water, part of the lubricant oil on the occlusive plate stays in a front portion within the crank chamber. As a result, the quantity of the lubricant oil returning to the oil pan decreases as much as the retained quantity. Thus, in order to prevent shortage of the supply amount of lubricant oil to portions to be lubricated, the conventional outboard engine has the need of using a large quantity of lubricant oil beforehand, and this forces to use a bulky oil pan and hence causes the outboard engine to be bulky and heavy. Furthermore, in a configuration where the crankshaft stirs the lubricant oil staying in the crank chamber, it invites an increase of the output loss of the engine. In addition, since a relatively large quantity of retained lubricant oil rushes to the opening immediately after the tilt-up is released during operation of the outboard engine, for the purpose of ensuring smooth outflow of lubricant oil from the crank chamber, the opening must be large, the occlusive plate inevitably becomes large, and these have encumbered realization of a compact, lightweight outboard engine.
The present invention has been made cognizing those problems in the background, and its main object is to provide a compact, lightweight outboard engine and prevent its output loss by substantially eliminating or minimizing the possibility of lubricant oil staying in the crank chamber during operation of the outboard engine in the tilt-up condition. Another object of the invention is to enable an inflow opening of the return oil path to be located in an optimum location.
According to the invention, there is provided an outboard engine having an engine body, an engine including a flywheel positioned at a lower end portion of a crankshaft extending vertically in the engine body and an oil pan positioned below the flywheel, a supply oil path for supplying lubricant oil released from an oil pump to a portion of the engine to be lubricated, and a return oil path for returning lubricant oil supplied to the portion to be lubricated back to the oil pan, and mounted to a boat stern with a mount device having a tilt shaft, characterized in that an upper wall of a flywheel chamber accommodating the flywheel is made up of a bottom wall of a crank chamber of the engine, the bottom wall having a front return oil path at a location forward of an inner circumferential surface of a circumferential wall of the flywheel chamber, the front return oil path being an oil path forming the return oil path to return lubricant oil from the crank chamber.
According to the invention, lubricant oil present in the crank chamber after lubricating portions of the engine to be lubricated flows down or drops onto the bottom wall of the crank chamber, then flows along the upper surface of the bottom wall forming the upper wall of the flywheel chamber, and flows into the return oil path, exiting from the crank chamber, until finally returning back to the oil pan 5. When the outboard engine is driven under a tilt-up condition, such as during cruising in shallow water, lubricant oil flowing on the bottom wall, then inclining down forward, flows into the front return oil path positioned forward of the inner circumferential wall surface of the circumferential wall of the flywheel chamber. Therefore, during operation under a tilt-up condition, it is possible to substantially prevent or minimize lubricant oil staying on the bottom wall. Also, immediately after the tilt-up condition is released, since substantially no lubricant oil or only an extremely small amount of lubricant oil stays in the cranks chamber, lubricant oil smoothly flows out from the crank chamber through the front return oil path.
As a result, the following effects are obtained. That is, when the outboard engine is in a tilt-up condition, since almost all of lubricant oil present on the bottom wall of the crankcase in the crank chamber flows into the front return oil path and finally returns back to the oil pan without staying on the bottom wall, it is possible to substantially prevent or minimize lubricant oil staying on the bottom wall. Therefore, unlike the conventional techniques, there is no need of increasing the quantity of lubricant oil retained in the oil pan, which will be required to be larger in capacity, taking account of the quantity of lubricant oil that will stay in the crank chamber. Accordingly, the oil pan can be decreased in size and weight, and the outboard engine can be decreased in size and weight as well.
Further, since it is substantially prevented that the crankshaft stirs lubricant oil staying in the crank chamber, output loss by agitation of lubricant oil can be prevented. Furthermore, since substantially no or only an extremely small amount of lubricant oil stays in the crank chamber, the front return oil path need not be increased in diameter for the purpose of ensuring smooth outflow of lubricant oil from the crank chamber including the lubricant oil having stayed there, immediately after the tilt-up condition is canceled, the front return oil path can be decreased in diameter in comparison to those of the conventional techniques, and the outboard engine can be made compact and lightweight so much.
Preferably, the circumferential wall is made up of double-wall portions and single-wall portions, a left wall portion and a right wall portion of the circumferential wall are made up of the single-wall portions, a front wall portion of the circumferential wall is made up of the double-wall portion having an inner wall and an outer wall, and the inner wall and the outer wall of the front wall portion define a space therebetween, in which the return oil path is formed.
According to this configuration, since the left wall portion and the right wall portion forming a part of the circumferential wall of the flywheel chamber are made up of single-wall portions, i.e. single-layered walls in the radial direction of the flywheel, the outer diameter of the flywheel chamber decreases in the right and left direction, and the front return oil path is formed in a space defined between the inner wall and he outer wall of the front wall portion. Thus the front return oil path can be made, making use of the circumferential wall of the flywheel chamber.
As a result, the following effects are obtained. That is, since the left wall portion and the right wall portion of the circumferential wall of the flywheel chamber are made up of single wall portions, the outer diameter of the flywheel chamber decreases in the right and left direction, and accordingly, the outboard engine decreases in width in the right and left direction, thereby contributing to making the outboard engine compact and increasing the freedom of location thereof on the boat stern. Furthermore, since the front return oil path is made by making use of the space between the inner wall and the outer wall of the front wall portion of the flywheel chamber, it is prevented that the bottom wall of the crank chamber becomes excessively large in the front and rear direction to make the front return oil path, and the outboard engine can be reduced in size and weight.
Preferably, the engine body in the outboard engine includes a cylinder block and a crankcase united to a front portion of the cylinder block to define the crank chamber, the bottom wall having formed the front return oil path being the bottom wall of the crankcase, an inner wall surface rising from an upper surface of the bottom wall of the crankcase cooperating with the upper surface of the bottom wall to define a projection space projecting forward in its plane view, and an inflow opening of the front return oil path opening in proximity of a rising start portion at a front-most portion of the projection space.
In this manner, because the inflow opening of the front return oil path made in the crankcase forming a front portion of the engine body opens in proximity of a rising start portion of the front-most portion of the projection space defined by the crankcase positioned in front of the engine body, when the outboard engine is driven under a tilt-up condition, lubricant oil flowing on the bottom wall then inclining down forward flows toward the front-most portion that is positioned in the lowest level, and flows into the inflow opening formed in proximity of the rising start portion of the front-most portion. As a result, a quantity of lubricant oil staying in the crank chamber is further reduced, and the effect of reducing the size and weight of the outboard engine and preventing the output loss is further enhanced.
The crankcase may have a front supply oil path formed to pass through the bottom wall to serve as an oil path forming the supply oil path, and the inflow opening may be located nearer to a reference plane including a rotation axis of the crankshaft and perpendicular to the center axis of the tilt shaft than the front supply oil path in the bottom wall of the crankcase.
In this manner, in the bottom wall of the crankcase, since the inflow opening is provided at a location nearer to the reference plane including the rotation axis of the crankshaft and perpendicular to the center line of the tilt shaft than the front supply oil path, without any restriction from the front supply oil path made in the bottom wall of the crankcase, the inflow opening is positioned at a location near to the reference plane where lubricant oil is likely to gather from peripheral portions distant from the referenced plane.
As a result, the following effects are obtained. That is, it is possible to select the best location for the inflow opening on the upper surface of the bottom wall of the crankcase, where lubricant oil is likely to flow in. That is, the inflow opening can be formed at an optimum location.
The outer circumferential wall of a pump body of the oil pump may makeup the circumferential wall throughout the entire circumference thereof, the engine body being united to a support portion formed as a part of a mount case via the outer circumferential wall at a coupling portion formed as a part of the engine body, and the coupling portion, the outer circumferential wall and the support portion being substantially equal in outer diameter.
In this manner, the structure substantially equalizing the outer diameter of the connecting portion from the support portion of the mount case, outer circumferential wall of the pump body of the oil pump to the coupling portion of the engine body to the outer diameter of the circumferential wall of the flywheel chamber produces the following effects. That is, in the outboard engine in which the engine body is united to the mount case through the pump body, since the outer diameter of the connecting portion from the support portion of the mount case to the coupling potion of the engine body can be minimized within a range sufficient for the pump body to accommodate the flywheel, the outboard engine can be further reduced in size and weight.